The present disclosure relates to an optical fiber coupler configured to couple signal beams from a plurality of input optical fibers into a non-circularly shaped output optical beam.
High-aspect ratio core (HARC) optical fibers are generally used for lasers and laser beam delivery applications. The HARC fiber has a characteristic feature: in one direction the beam traveling through the HARC fiber has close to diffraction-limited beam quality, and in the perpendicular direction the beam has either a multi-mode quality or also close to diffraction-limited beam quality, depending on the design of particular fiber.
FIG. 1 shows a conventional arrangement that is configured to increase the brightness of a fiber array. As shown in FIG. 1, micro-lens array 100 is positioned near a linear array of fibers 200 to eliminate the spacing between fibers 200a of array 200 and thereby to increase the brightness of fiber array 200. An array of lenses 100 is configured to optically remove the space taken up by claddings 200b, thereby to form an optical equivalent of an array of cores without the claddings.
In the arrangement shown in FIG. 1, the center-to-center spacing between fibers 200a is represented as pitch p, the core size of each fiber 200a is represented as d, the raw beam divergence is represented as θraw, and the magnification is represented as M.
As shown in FIG. 1, fibers 200a are aligned with their exit facets in a single plane, and lens array 100 is positioned away from fibers 200a at a distance that is equal to the focal length of the lenses. The beams propagating through lenses 100a are parallel to each other. If the lens apertures are designed to properly account for magnification M, the resulting output beams may have quite small spacings between them corresponding to an increased brightness relative to that of the initial fiber array 200.
The approach shown and described with respect to FIG. 1 suffers from a number of challenges relating to optical fabrication and positioning tolerances, which will affect the final collimation and parallelism of the output beams.
In another approach designed to improve the brightness of fiber array 200, an array of quarter-pitch Gradient Index (GRIN) lenses (not shown) may be used instead of micro-lens array 100 of FIG. 1. The array of quarter-pitch GRIN lenses may be fused between the individual input fibers and the HARC output fiber (not shown). However, this approach, like the approach described with respect to FIG. 1, does not cure the problems of optical fabrication and positioning tolerances.
Applicant has identified a need for an all-fiber device that is capable of coupling optical signal(s) from a number of fiber-coupled devices into a non-circularly shaped output optical beam which can be coupled into the device with non-circularly shaped waveguide (e.g., HARC fiber, planar waveguide, etc) or can be used as a free-space beam. Such an all-fiber coupler device needs to maintain the beam quality while improving brightness.